From: Dwayne Grant McConnell <decimal@us.ibm.com>We need to check the channel count of the signal notification registersbefore reading them, because it can be undefined when the count iszero. In order to read count and data atomically, we read from thesaved context.

This patch uses spu_acquire_saved() to force a context save before a/signal1 or /signal2 read. Because of this it is no longer necessary tohave backing_ops and hw_ops versions of this function so they have beenremoved.

Regular applications should not rely on reading this registerto be fast, as it's conceptually a write-only file from the PPEperspective.